304 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



In Lillie's words, " In the presence of either acid or alkali the osmotic 

 pressure of the gelatin thus shows a marked increase, which, within the 

 above range of concentrations, exhibits a certain proportionality to the 

 quantity of acid or alkali added. For equivalent concentrations acid 

 produces a somewhat greater increase than alkali. The change in 

 osmotic properties is to be attributed to a finer subdivision of the col- 

 loidal particles and a consequent increase in the, surface of intersec- 

 tion between colloidal particles and medium." The osmotic pressure of 

 gelatin and of egg-albumin is unaffected by the addition of non-electro- 

 lytes, such as cane-sugar, glucose, glycerol and urea, but is considerably 

 affected by the addition of Inorganic Salts, being depressed thereby. 

 The decrease of the osmotic pressure exerted by the protein depends 

 upon the nature of both the anion and the cation of the added salt. 

 The depression increases in the order: Alkali metals < alkaline 

 earths < heavy metals (for cations); and CNS<I<Br<NO 3 <Cl<F< 

 plurivalent anions, SCX, tartrate, citrate, phosphate (for anions). 

 This fact is especially significant when we recollect that this is the 

 order in which the various ions bring about the dehydration and coagu- 

 lation of proteins (see Chapter VIII). 



THE SWELLING OF PROTEIN JELLIES. 



The proteins, as we have seen, exert a small, but a definite osmotic 

 pressure. They are at the same time not diffusible through jellies 

 or only very slightly so. Any crystalloid which may chance to be 

 present in the external fluid which bathes a Gelatin plate, therefore, 

 can penetrate the gelatin, although perhaps more slowly than water. 

 The gelatin from the interior of the plate cannot similarly escape into 

 the surrounding medium. The gelatin plate, therefore, acts like an 

 osmometer which provides its own membrane which is permeable for 

 water and crystalloids but not for colloids. Hence, when dry gelatin 

 or concentrated jellies are placed in water they take up water and 

 increase in volume. 



A phenomenon in the domain of crystalloids which presents some 

 analogies to the swelling of colloidal jellies is the following : If we place 

 at the bottom of a column of distilled water a layer of Phenol and intro- 

 duce below this a layer of saturated solution of potassium chloride in 

 water and now allow the system to stand at constant temperature, the 

 layer of phenol gradually moves up the column of water, in other 

 words the layer of solution below the phenol "swells." The solvent, 

 water, being soluble in phenol, the phenol is permeable by it, while the 

 potassium chloride, being insoluble in phenol, cannot pass through the 

 supernatant layer of phenol. 



Not only osmotic, but also chemical phenomena must, however, 

 play a part in the swelling of protein jellies. As we have seen in 

 Chapter VIII the passage of a protein into solution involves the addi- 

 tion of the elements of water to terminal NH 2 and COOH groups 



